Progressive cavity pumps have been used for decades to pump fluids from an oil well to the surface through a production tubing string. A progressive cavity pump is conventionally powered by a rotating sucker rod string positioned within the tubing string.
It is periodically desirable to retrieve a downhole pump to the surface for inspection and/or repair. As a practical matter, it is highly preferred to retrieve the downhole pump without requiring the retrieval of the tubing string. Many operations also allow sand or other soil material from the formation to enter the interior of the pump, thereby adversely affecting pumping efficiency. In a flushing operation, the rotor may be axially pulled from the stator housing, the interior of the housing flushed to remove the sand or other debris, then the rotor reinserted into the housing to continue pumping operations.
An existing downhole pumping system includes a drive coupling at the upper end of the rotor which may engage a lifting nut to disengage a pump from a landing nipple and retrieve the pump to the surface. A flat shoulder on the lifting nut may thus engage a similar flat surface at the upper end of the drive coupling. The drive coupling outer diameter is larger than the lifting nut internal diameter, such that when the two components engage during pump retrievable, the pump housing is unseated from the landing nipple and retrieved to the surface.
In order to conduct a “flush by” operation, the rotor is axially pulled from the stator and flushed with clean fluid. During this pulling operation, the pump stator should remain seated in the landing nipple, however the drive coupling may engage the lifting nut and unseat the entire pump. One existing solution is to add an extension between the top of the stator and the lifting nut which is sufficiently long to allow the rotor to be pulled out of the stator before engaging the lifting nut. Sucker rods typically have a maximum length of 30 feet, and if the pump length is less than 30 feet, adding such an extension may allow the rotor to be pulled out of the stator before engaging the lifting nut. This solution increases the length of the pumping system, but may work in cases where an extension tube is less than 30 feet in length to allow flush by to be performed. For other applications, an extension greater than 30 feet is not a practical solution to the problem because a sucker rod coupling positioned along the length of this 30 feet extension may engage the inside diameter of the lifting nut and still inadvertently unseat the pump. One possibility is a single-length sucker rod which has a length of 50 feet or more without a coupling, but this solution would be expensive. It would also be difficult to transport such a special sucker rod, and would require a full size workover rig to perform a flush by operation. The cost of a single 50 foot long sucker rod may thus be prohibitive.
A progressive cavity pump with a retrievable rotor is disclosed in U.S. Pat. No. 5,988,992. A retrievable electric pump is disclosed in U.S. Pat. No. 5,954,483. U.S. Pat. No. 6,675,902 discloses a type of progressive cavity pump, and U.S. Pat. No. 6,089,832 discloses a retrievable electric motor pump. U.S. Pat. No. 5,871,051 discloses a retrievable rotary pump, and U.S. Pat. No. 6,695,060 discloses another type of retrievable progressive cavity pump. Improved pump construction is disclosed in U.S. Pat. No. 6,981,045.
The disadvantages of the prior art are overcome by the present invention, and an improved pumping system for retrieving a downhole pump is hereinafter disclosed, wherein a pumping system allows the rotor to be pulled from the stator for reliably performing a flushing operation.